1. Field of the Invention
The present invention relates to a laser light wavelength shifter and more particularly, a laser light wavelength shifter for shifting a laser light emitted from a semiconductor laser to a shorter wavelength laser light.
2. Description of the Related Art
Semiconductor lasers are commonly employed as the laser light source for use in various types of laser apparatuses which have to be compact in size for matching their applications. Also, it is desired that the laser light on each application apparatus has a shorter wavelength for attaining optimum performance.
For example, a recording capability allowing more data to be written into and read out from a given area of a recording medium has acquired a considerable value added in the case of an optical disk system, a laser printer, a laser scanner, and other optoelectronic apparatuses. For this purpose, it is necessary to converge a laser light into a spot of a smaller diameter. When the wavelength of a laser light is .lambda. and the angle of convergence is .theta., the diameter of a laser spot .phi. is obtained from: EQU .phi.=2.lambda./.pi.tan.phi.
As understood, the smaller the laser wavelength .lambda., the smaller the spot diameter .phi. is focused. Thus, a shorter wavelength of laser light is needed for enhancement in the convergence of a laser light.
A laser source for producing a shorter wavelength of laser light is known as, e.g., an He-Cd laser, an excimer laser, or an Ar laser, each of which is too large in size to be successfully installed in any of the aforementioned laser apparatuses.
A semiconductor laser, which is a small-sized laser source, is eligible for installation in the laser application apparatuses described above. However, a technique of crystalline process required for producing such a semiconductor laser as satisfying the foregoing requirement has not yet been established. It will take some time before an improved laser producing a shorter wavelength of laser light is developed. So far, some laser light wavelength shifters have been proposed for shortening the wavelength of a laser light emitted from a known semiconductor laser to one half with the use of a second harmonics generator (abbreviated as SHG). An example of prior art laser light wavelength shifters will now be described.
A prior art shifter shown in FIG. 1 employs a single-mode semiconductor laser medium 31 as the light source for producing a single-mode fundamental wave which is essential for wavelength shift at high efficiency. In operation, a laser light emitted from the semiconductor laser 31 is converged by a condenser lens 32 onto the incident end of an SHG 33 formed of non-linear optical crystal using Cerenkov phase matching. Then, the SHG 33 decreases the wavelength of the laser light to a half.
Although the arrangement of the above-described shifter is simple, the single-mode semiconductor laser employed fails to produce a high power output of a single wavelength laser light which is as high as more than several hundreds of milliwatts. Accordingly, the efficiency of wavelength conversion stays low and no high power output of a wavelength shortened laser light (referred to as a frequency-doubled wave) will be obtained. Also, the frequency-doubled wave of Cerenkov phase matching propagates in a cone shape. Hence, a specific optical system is needed for convergence of the frequency-doubled wave.
Another prior art shifter shown in FIG. 2 has a multi-mode semiconductor laser 41 which serves as an exciting source and produces a high power of laser light. In operation, a laser light emitted from the multi-mode laser 41 is directed by a collimating lens 42 and dichroic mirror 43 to a solid laser medium 44 for excitation. Then, a solid laser resonator comprising the dichroic mirror 43, the solid laser medium 44, lenses 45 and 47, and a dichroic mirror 48 is activated for having a beam profile of appropriate convergence on an SHG crystal 46. As the result, a shifted light of which wavelength is a half of that of the input laser light is propagated from the SHG crystal 46 through the lens 47 and the dichroic mirror 48 for output.
As the wavelength shifting is carried out at two steps, the entire arrangement of this shifter becomes intricate. Also, a resultant frequency-doubled wave is more than 500 nm in the wavelength although its power is increased to a few milliwatts. The efficiency of shifting the output of a semiconductor laser is as low as about 1%. Hence, the laser has to be employed of high power supply, high power output type.